1. Field
Exemplary embodiments of the present invention relate to a semiconductor design technology, and more particularly, to an output circuit of semiconductor device.
2. Description of the Related Art
In general, in a low-power semiconductor device (i.e., designed to operate under low power condition), an output circuit is desirable to reduce power consumption, and due to this fact, it may be difficult to secure signal integrity of output signal.
For example, in a main memory or a graphic memory, which has no limitation in terms of power consumption, a termination control circuit such as an on-die termination (ODT) circuit is included to be connected to a data output terminal to suppress reflective waves when transmitting data, thereby securing the signal integrity of output data signal. However, although such termination control circuit provides an advantage in that the reflective waves may be effectively removed, constant DC current may be required to be consumed by termination resistor. Due to such a concern, the termination control circuit may not be employed to a semiconductor device in a mobile system, which should operate under low power condition.
FIG. 1 is a block diagram illustrating a data output circuit of a conventional low-power semiconductor device.
Referring to FIG. 1, the data output circuit includes a pull-up operation block 100, and a pull-down operation block 120.
The pull-up operation block 100 drives an output node DQ to a logic high level, corresponding to a power supply voltage (VDD), based on a logic high level of an input signal IN_SIG.
The pull-down operation block 120 drives the output node DQ to a logic low level, corresponding to a ground voltage (VSS), based on a logic low level of the input signal IN_SIG.
The pull-up operation block 100 includes a plurality of pull-up driving units 104, 105, 106 and 107. Operations of the plurality of pull-up driving units 104, 105, 106 and 107 are on/off controlled based on driving force control codes SR_CTRL<1:4>. For reference, while it is exemplified in the drawing for the sake of convenience in explanation that a first pull-up driving unit 104, a second pull-up driving unit 105, a third pull-up driving unit 106 and a fourth pull-up driving unit 107 are included in the plurality of pull-up driving units 104, 105, 106 and 107, configuration may be actually made such that an increased or decreased number of pull-up driving units may be included in the plurality of pull-up driving units 104, 105, 106 and 107. Also, while it is exemplified that the driving force control codes SR_CTRL<1:4> are a signal which is constituted by 4 bits, it is to be noted that driving force control codes may actually be a signal which is constituted by an increased or decreased number of bits.
The pull-down operation block 120 includes a plurality of pull-down driving units 124, 125, 126 and 127. Operations of the plurality of pull-down driving units 124, 125, 126 and 127 are on/off controlled based on the driving force control codes SR_CTRL<1:4>. For reference, while it is exemplified in the drawing for the sake of convenience in explanation that a first pull-down driving unit 124, a second pull-down driving unit 125, a third pull-down driving unit 126 and a fourth pull-down driving unit 127 are included in the plurality of pull-down driving units 124, 125, 126 and 127, configuration may be actually made such that an increased or decreased number of pull-down driving units may be included in the plurality of pull-down driving units 124, 125, 126 and 127. Also, while it is exemplified that the driving force control codes SR_CTRL<1:4> are a signal which is constituted by 4 bits, it is to be noted that driving force control codes may actually be a signal which is constituted by an increased or decreased number of bits.
The reason why the plurality of pull-up driving units 104, 105, 106 and 107 are included in the pull-up operation block 100 and the plurality of pull-down driving units 124, 125, 126 and 127 are included in the pull-down operation block 120 as in the above-described configuration resides in that, because the conventional semiconductor device is a low-power semiconductor device, in which a separate termination control circuit such as an ODT circuit may not be included. That is to say, instead of controlling the impedance matching of the output node DQ through a separate termination control circuit such as an ODT circuit, the impedance matching of the output node DQ is controlled through a scheme of controlling the driving force of an output driver. As a scheme for controlling the driving forces of the pull-up operation block 100 and the pull-down operation block 120, a method of controlling the value of the driving force control codes SR_CTRL<1:4> is used as exemplified in the following descriptions. The values of the driving force control codes SR_CTRL<1:4> may be defined in advance through an operation setting circuit in the semiconductor device, such as a memory register set (MRS), or by receiving values from an outside of the semiconductor device.
For example, it is assumed that output impedance of the output node DQ in view of single driving unit included in the pull-up operation block 100 and the pull-down operation block 120 is 120Ω. when only the first bit of the driving force control code SR_CTRL<1> is activated and the second to fourth bits of the driving force control codes SR_CTRL<2:4> are deactivated and thus only the first pull-up driving unit 104 operates and the second to fourth pull-up driving units 105, 106 and 107 do not operate, the output impedance of the output node DQ becomes 120Ω. When only the first and second bits of the driving force control codes SR_CTRL<1:2> are activated and the third and fourth bits of the driving force control codes SR_CTRL<3:4> are deactivated among the driving force control codes SR_CTRL<1:4> and thus only the first and second pull-up driving units 104 and 105 operate and the third and fourth pull-up driving units 106 and 107 do not operate, the output impedance of the output node DQ becomes 60Ω. When only the first to third bits of the driving force control code SR_CTRL<1:3> are activated and the fourth bit of the driving force control code SR_CTRL<4> is deactivated, and thus the first to third pull-up driving units 104, 105 and 106 operate and the fourth pull-up driving unit 107 does not operate, the output impedance of the output node DQ becomes 40Ω. When all the bits of the driving force control codes SR_CTRL<1:4> are activated, and thus all the first to fourth pull-up driving units 104, 105, 106 and 107 operate, the output impedance of the output node DQ becomes 30Ω.
In the same manner, it is assumed that output impedance of the output node DQ in view of single driving unit included in the pull-down operation block 120 is 120Ω, when only the first bit of the driving force control code SR_CTRL<1> is activated and the second to fourth bits of the driving force control code SR_CTRL<2:4> are deactivated and thus only the first pull-down driving unit 124 operates and the second to fourth pull-down driving units 125, 126 and 127 do not operate, the output impedance of the output node DQ becomes 120Ω. When only the first and second bits of the driving force control code SR_CTRL<1:2> are activated and the third and fourth bits of the driving force control code SR_CTRL<3:4> are deactivated among the driving force control codes SR_CTRL<1:4>, and thus only the first and second pull-down driving units 124 and 125 operate and the third and fourth pull-down driving units 126 and 127 do not operate, the output impedance of the output node DQ becomes 60Ω. When only the first to third bits of the driving force control code SR_CTRL<1:3> are activated and the fourth bit of the driving force control code SR_CTRL<4> is deactivated and thus only the first to third pull-down driving units 124, 125 and 126 operate and the fourth pull-down driving unit 127 does not operate, the output impedance of the output node DQ becomes 40Ω. When all the bits of the driving force control codes SR_CTRL<1:4> are activated, and thus all the first to fourth pull-down driving units 124, 125, 126 and 127 operate, the output impedance of the output node DQ becomes 30Ω.
However, when the impedance matching of the output node DQ is controlled by controlling a driving force of an output driver as in the above-described conventional semiconductor device, a slope is likely to become gentle due to inter-symbol interference (hereafter, referred to as ‘ISI’) among output signals, and a problem may be caused in that a data eye is narrowed or is even closed.
FIG. 2 is of diagrams for explaining ISI occurred in the data output circuit shown in FIG. 1.
Referring to FIG. 2, in the conventional data output circuit of a low-power semiconductor device, as the output impedance of the output node DQ for impedance matching is small, ISI occurs less and a slope retains a relatively steep state, and due to this fact, a data eye is sufficiently secured. On the contrary, as the output impedance of the output node DQ for impedance matching is large, ISI occurs more and a slope retains a relatively gentle state, and due to this fact, a data eye is not sufficiently secured.
In detail, when the output impedance of the output node DQ for impedance matching is 34Ω, a data eye becomes 471 p and a data eye is sufficiently secured.
When the output impedance of the output node DQ for impedance matching is 40Ω, a data eye becomes, for example, ‘329 ps’. When the output impedance of the output node DQ for impedance matching is 48Ω, a data eye becomes ‘257 ps’. When the output impedance of the output node DQ for impedance matching is 60Ω, a data eye becomes ‘25.8 ps’. Thus, as the output impedance of the output node DQ increases, the magnitude of a data eye abruptly decreases.
As a consequence, the fact that a data eye of a signal outputted from a data output circuit is not sufficiently secured is directly connected with a problem of deteriorating the reliability of the output signal, which leads to a problem that a data output operation may not be normally performed. Further, because these problems exert greater influences when a low-power semiconductor device is to operate at a high speed, a serious concern may be caused when considering a technology trend toward a high speed semiconductor device.